The worrisome thing about this is that it sort of also suggests Kirk never spent more than five years out there dealing with V'Ger-like unknowns. Did his entire Starfleet career indeed consist of the TOS five-year mission, plus desk work?
That doesn't ring true at all. Kirk went to strange new worlds in his youth, too (say, "A Private Little War" backstory), even if his earliest years were spent doing instructor work at the Academy.
I guess we have to interpret Kirk's TMP words as "five years out there dealing with unknowns like that in the role of a starship CO", as opposed to "in the role of a starship security officer" or whatever. Which still isn't completely satisfactory, because it suggests Kirk never commanded any starship before TOS. Or then we can narrow it down further, with Kirk saying "five years out there in truly deep space" which rules out his previous commands of lesser starships or other such spacecraft. But if we keep on doing that, we can basically water down all of Kirk's rambling and remove all power of evidence from the statement. Say, perhaps Kirk only counts his second five-year mission because the first one was such a let-down and doesn't look good in his resume?
Why is the idea that Enterprise was Kirks first command unsatisfactory? there seems to be no reason why it wouldn't be. Sulu's first command is the Excelsior which is a pretty important vessel by Starfleets standards and one of the whole reasons Kirk was able to take command from Decker was that Decker, who was about to captain the refit of Enterprise ( again very high prestige one assumes ) had no actual command experience.
Seem that Starfleet is happy to put young and untested commanders in charge of important vessels.